I am ready to train my crew to 2nd skill. Should I go for BIA or more indivdual perks? Is BIA usefull or is it no noticeable? This crew is currently serving my AMX M4 1945 and are quite near to be retrained to AMX 50 100.

BIA works same as vents. If you used vents on ur tanks and noticed improvement, than you will notice it with BIA too.
You can use it instead of vents(so you can put some other module) or with vents as additional boost.
It influences all crew skills(aim time, repairs, camouflage.....) don't know are u gonna notice that, but u will sure notice faster reload.

The AMX-50 series can't use a rammer to expedite reload anyway, so might as well use that slot for vents and throw BIA on top of that for extra assist. Although the combo only takes a few seconds off the ghastly, ghastly 50-sec magazine reload the 100 has with the eponymous gun... >.<

BIA increases crew skills by 5%, which means using WoT algorithms a 2% increase in your tank stats. Reload is just 2% faster, you get 2% more traverse etc. I don't think BIA is worth all the skill slots, you could and should use the slots for other perks instead.

Drop skills and perks, choose BIA as first skill on all crew members, take whatever skill you want as the second one. That way you don't spend any time with perks that do nothing because they are not 100%

In my russian tanks I use BiA in combination with vents. I take BiA first and then proceed to other skills/perks, depending on the tank. I'm aware that even with vents the edge you gain is little, but I want that edge anyway. Might save your ass sometimes.

bia reduces the reload time, in a batchat vs batchat i killed him because i could reload faster than him(first reload from the beginning of the fight), it just will take some time but you will have the second perk when you reach the 50b.

Pick a skill rather than a perk, then once the skill is trained to 100%, retrain them with the perk you wanted.

You lose a little xp, but it means you have them learning something they can use during the grind (as opposed to skills, which are useless until you hit 100% with them)

This.
When you're training a new secondary always pick something that improves as you go. Once you've trained it to just over 100% switch it out for something that doesn't start working till it gets to 100%.
If I was thinking if using BIA as my second secondary I'd retrain my first one to it instead so I received the benefits straight away and retrain repair or camo from scratch.